Add instructions about how to use

This was copied almost exactly from the page I wrote for the Causes
style guide.

Change-Id: I09f2b0dacdb584f00b6adbe57579dd662074f779
Reviewed-on: https://gerrit.causes.com/31486
Reviewed-by: Joe Lencioni <joe.lencioni@causes.com>
Tested-by: Joe Lencioni <joe.lencioni@causes.com>